The present invention relates to multijunction solar cells, and in particular to high efficiency solar cells comprised of III-V semiconductor alloys.
Multijunction solar cells made primarily of III-V semiconductor alloys are known to produce solar cell efficiencies exceeding efficiencies of other types of photovoltaic materials. Such alloys are combinations of elements drawn from columns III and V of the standard Periodic Table, identified hereinafter by their standard chemical symbols, names and abbreviation. (Those of skill in the art can identify their class of semiconductor properties by class without specific reference to their column.) The high efficiencies of these solar cells make them attractive for terrestrial concentrating photovoltaic systems and systems designed to operate in outer space. Multijunction solar cells with efficiencies above 40% under concentrations equivalent to several hundred suns have been reported. The known highest efficiency devices have three subcells with each subcell consisting of a functional p-n junction and other layers, such as front and back surface field layers. These subcells are connected through tunnel junctions, and the dominant layers are either lattice matched to the underlying substrate or are grown over metamorphic layers. Lattice-matched devices and designs are desirable because they have proven reliability and because they use less semiconductor material than metamorphic solar cells, which require relatively thick buffer layers to accommodate differences in the lattice constants of the various materials. As set forth more fully in U.S. patent application Ser. No. 12/217,818, entitled “GaInNAsSb Solar Cells Grown by Molecular Beam Epitaxy,” which application is incorporated herein by reference, a layer made of GaInNAsSb material to create a third junction having a band gap of approximately 1.0 eV offers a promising approach to improving the efficiency of multijunction cells. Improvements are nevertheless to be considered on the cell described in that application.
The known highest efficiency, lattice-matched solar cells typically include a monolithic stack of three functional p-n junctions, or subcells, grown epitaxially on a germanium (Ge) substrate. The top subcell has been made of (Al)GaInP, the middle one of (In)GaAs, and the bottom junction included the Ge substrate. (The foregoing nomenclature for a III-V alloy, wherein a constituent element is shown parenthetically, denotes a condition of variability in which that particular element can be zero.) This structure is not optimal for efficiency, in that the bottom junction can generate roughly twice the short circuit current of the upper two junctions, as reported by J. F. Geisz et al., “Inverted GaInP/(In)GaAs/InGaAs triple junction solar cells with low-stress metamorphic bottom junctions,” Proceedings of the 33rd IEEE PVSC Photovoltaics Specialists Conference, 2008. This extra current capability is wasted, since the net current must be uniform through the entire stack, a design feature known as current matching.
In the disclosure of above noted U.S. patent application Ser. No. 12/217,818, it was shown that a material that is substantially lattice matched to Ge or GaAs with a band gap near 1.0 eV might be used to create a triple junction solar cell with efficiencies higher than the structure described above by replacing the bottom Ge junction with a junction made of a different material that produces a higher voltage.
In addition, it has been suggested that the use of this 1 eV material might be considered as a fourth junction to take advantage of the entire portion of the spectrum lying between 0.7 eV (the band gap for germanium) and 1.1 eV (the upper end of the range of bandgaps for the ˜1 eV layer). See for example, S. R. Kurtz, D. Myers, and J. M. Olson, “Projected Performance of Three and Four-Junction Devices Using GaAs and GaInP,” 26th IEEE Photovoltaics Specialists Conference, 1997, pp. 875-878. Ga1-xInxNyAs1-y has been identified as such a 1 eV material, but currents high enough to match the other subcells have not been achieved, see, e.g., A. J. Ptak et al., Journal of Applied Physics 98 (2005) 094501. This has been attributed to low minority carrier diffusion lengths that prevent effective photocarrier collection. Solar subcell design composed of gallium, indium, nitrogen, arsenic and various concentrations of antimony (GaInNAsSb) has been investigated with the reported outcome that antimony is helpful in decreasing surface roughness and allowing growth at higher substrate temperatures where annealing is not necessary, but the investigators reported that antimony, even in small concentrations is critical to be avoided as detrimental to adequate device performance. See Ptak et al., “Effects of temperature, nitrogen ion, and antimony on wide depletion width GaInNAs,” Journal of Vacuum Science Technology B 25(3) May/June 2007 pp. 955-959. Devices reported in that paper have short circuit currents far too low for integration into multijunction solar cells. Nevertheless, it is known that Ga1-xInxNyAs1-y-zSbz with 0.05≦x≦0.07, 0.01≦y≦0.02 and 0.02≦z≦0.06 can be used to produce a lattice-matched material with a band gap of approximately 1 eV that can provide sufficient current for integration into a multijunction solar cell. However, the voltages generated by subcells containing this material have not exceeded 0.30 V under 1 sun of illumination. See D. B. Jackrel et al., Journal of Applied Physics 101 (114916) 2007. Thus, a triple junction solar cell with this material as the bottom subcell has been expected to be only a small improvement upon an analogous triple junction solar cell with a bottom subcell of Ge, which produces an open circuit voltage of approximately 0.25 V. See H. Cotal et al., Energy and Environmental Science 2 (174) 2009. What is needed is a material that is lattice-matched to Ge and GaAs with a band gap near 1 eV that produces an open circuit voltage greater than 0.30 V and sufficient current to match (Al)InGaP and (In)GaAs subcells. Such a material would also be advantageous as a subcell in high efficiency solar cells with 4 or more junctions.